(a) Field of the Invention
The present invention relates to a bulb for selectively radiating light of desired wavelength utilizing interference and, more particularly, to a bulb for selectively projecting to the front only visible light among light components radiated from a filament.
(b) Description of the Prior Art
A lamp is conventionally known in which a visible light reflecting/infrared light transmitting layer is formed on a reflecting surface of a reflecting bulb. The visible light reflecting/infrared light transmitting layer consists of a total of seven to nine alternately formed thin titanium oxide layers and thin silica layers having a lower refractive index than that of the thin titanium oxide layers. This lamp reflects toward the front of its reflecting surface visible light from the light radiated from a filament or a light-emitting tube as a light-emitting member. The lamp transmits the infrared light through the reflecting surface toward the back. Therefore, the lamp can project light having a small infrared ray component.
A high-efficiency bulb has also been proposed recently. This bulb has a filament as a light-emitting member arranged at the center of a T-shaped or tubular bulb. A visible light transmitting/infrared light reflecting film is formed on at least one of the inner and outer surfaces of the bulb. The visible light transmitting/infrared light reflecting film also consists of a total of seven to nine thin titanium oxide layers and thin silica layers of a lower refractive index which are formed alternately. Of the light emitted from the filament, the visible light is transmitted through the reflecting film and radiated to the outside. Infrared light is reflected by the reflecting surface and is fed back to the filament, thereby heating the filament. This bulb therefore has a high efficiency and can radiate light having a small infrared component.
Both the visible light reflecting/infrared light transmitting and the visible light transmitting/infrared light reflecting film comprise metal oxide layers of a high refractive index and metal oxide layers of a low refractive index which are formed alternately. Both these films utilize interference to achieve the prescribed effect. Such films have different transmitting or reflecting wavelength regions in accordance with the thickness of each layer.
An optical film having such characteristics will be referred to as an optical interference film herein.
Conventional methods of forming thin titanium oxide layers include a method of directly forming a titanium oxide layer or the like on a bulb surface or a thin silica layer by the vacuum deposition method, the sputtering method, or the CVD method; or a method of coating a solution of an organic titanium compound by the spraying method, the spinner method, the dipped coating method, the brush coating method, or the printing method, and thermally decomposing the coated film into titanium oxide. Among such methods, the coating method is preferred for mass production. The coating method uses as a coating solution an organic solvent solution of a titanium alkoxide having a general formula Ti(OR).sub.4 (where R is an alkyl group), such as tetraisopropoxy titanate or tetrabutoxy titanate. However, titanium alkoxides are easily hydrolyzed upon absorption of water in the air. For this reason, the coating solution easily becomes turbid or highly viscous; it has a poor stability and is hard to handle.
In order to solve this problem, a stable coating solution of a titanium alkoxide has been proposed which uses as a solvent a chelating agent such as acetylacetone or methyl acetoacetate, or an acetec ester of an alcohol. However, with this method, although the humidity resistance of the coating solution is improved, the film formation performance of the thin titanium oxide layer is poor and the obtained film has a small refractive index. Another coating solution has also been proposed which uses an organic solvent solution containing a polymer obtained by polymerizing a water-containing titanium alkoxide. Although this coating solution has a good film formability of a thin titanium layer, it is still subject to turbidity due to the influence of humidity. Japanese Patent Disclosure No. 54-43241 proposes still another coating solution which is obtained by polymerization by adding water to a titanium alkoxide and stabilizing the solution by adding a chelating agent such as acetylacetone. This coating solution has a good stability against high humidity and has a good film formability when only a single thin titanium oxide layer is formed. However, when this coating solution is used to form a multilayered film of thin metal oxide layers of a low refractive index and thin silica layers as in the case of an optimal interference film, it has poor adhesion between the thin titanium oxide layers and the thin metal oxide layers of low refractive index.
When a thin titanium oxide layer is used as an optical interference film, its refractive index greatly affects the optical characteristics of the film. More specifically, an optical interference film generally comprises low refractive index layers and high refractive index layers formed alternately. The optical characteristics of such a film change in accordance with a ratio of the refractive index of the low refractive index layers to that of the high refractive index layers. The higher the ratio, the higher the reflectance and the wider the reflecting wavelength range. For this reason, titanium oxide layers as high refractive index layers preferably have a higher refractive index. However, when conventional organic titanium compounds are used to obtain an optical interference film on a glass bulb having a high refractive index by adjusting the composition and thermal decompositon conditions, the film becomes turbid due to a temperature increase when the lamp is ON. This is caused since the crystal structure of titanium oxide changes from the anatase phase to the rutile phase. The turbidity of a film significantly degrades the quality of the film as an interference film due to light scattering. Upon this phase transformation, cracks are formed in addition to an increase in turbidity and the film is easily separated when the lamp is turned on and off. The temperature at which the film is subjected to this phase transformation is different in accordance with the original material used and is 600.degree. to 700.degree. C. when the above solution is used. Therefore, phase transformation is an important problem in the bulb which is operated at a high temperature.